The present invention relates to a digital signal demodulator, and more particularly to a demodulator for demodulating a modulated signal which has been reproduced from a recording medium, i.e., a magnetic tape on which signals are recorded at a standard tape speed in conformity with the industry standards for R-DAT, so that the R-DAT, i.e., a rotary magnetic head type digital audio tape recorder, performs a data corrective operation using a parity system to obtain a correct demodulated data.
Various kinds of digital signal recording and reproducing equipment or digital signal reproducing equipment for reproducing purpose have been conventionally known, in which an analog audio signal is subject to digital modulation such as pulse code modulation (PCM) conforming to a known code format and is recorded on a medium. The recorded PCM signal is subject to reproduction when desired. In such a digital signal having been already recorded on the recording medium to be reproduced by digital signal recording/reproducing equipment or digital signal reproducing equipment, various parity codes are included in order to detect a code error and correct them for restoring the original audio signal even if there exist code errors in the reproduced data due to signal drop out etc. at the time of reproducing. Thus, a parity operation using a parity code added at the time of recording and the data from which the added parity is derived is performed to detect an error of the data to further perform a predetermined corrective operation, thus to correct and restore the original audio signal.
However, the following problem cannot be avoided in such conventional equipment. Also when a lack in a portion of the reproduced signal occurs as drop out etc. due to a surface irregularity of the magnetic tape for example, at the time of reproduction, a noise occurring during the signal lacking period is outputted from a demodulating circuit as a random data signal. Accordingly, there are instances where the random data (noise) is accidentally reproduced as correct data in the parity operation of the error detection and correcting circuit of the subsequent stage, although the data concerned are false data caused by a random data pattern which takes place.
Particularly, in the rotary head type digital audio tape recorder (which will be abbreviated as "R-DAT" hereinafter), since a simple parity system is adopted, namely the parities derived from two symbols of an ID code and a block address only are added to the head portion of each data block, there is a high possibility that the data would be determined as correct data although those data are false in the case of a random error pattern due to the noise.
As described later in detail the R-DAT system employs a subcode which includes 8 symbols as a "pack", and the last symbol in the pack is a parity symbol of a simple parity system derived from the remaining 7 symbols. Also included in the subcode is an error correction code Cl as one of the 8 packs therein. However, in the high tape speed data search mode, or in the case where the error correction code Cl is not used for the purpose of reducing circuit complexity, detecting subcode data errors is solely dependent upon the above described parity symbol. If this is the case, chances are high that false data could be erroneously judged as correct data.